1. Field of the Invention
This patent specification generally relates to downhole fluid analysis and in-situ formation evaluation. More particularly, this patent specification relates to in-situ evaluations of reservoir sanding and fines migrations.
2. Background of the Invention
The design of the completion of a producing well is a complex process that uses multiple sources of reservoir information. Similarly, the design of the production system, including artificial lift and surface facilities, relies on such information. A common problem for many wells is the tendency to produce solid particles from the reservoir formation, such as sand grains, fine particles, and the like. The production of solid particles is usually termed “sanding”, although the particles need not be sand; for example, a carbonate reservoir that produces solid particles is said to produce “sand”. Known methods for predicting sanding potential include using a stress-based mechanical model of the formation. Important inputs to such models are pore pressure, stress conditions, rock strength, and rock material properties. Rock material properties include grain sorting, shape, and size distribution.
The rock material properties are typically determined from mechanical testing on reservoir core samples. The tests are conducted at a surface laboratory. However, once a core is extracted from underground, it is impossible to restore it to exactly the same stress state as existed in the reservoir. Furthermore, it is possible for the core sample to undergo irreversible changes before it can be tested, including total collapse of the core. Although techniques exist for in-situ measurement of rock properties, these are limited mainly to measuring stress conditions. M. A. Addis et al. discuss a “Sand Influx Test” in which sand is deliberately produced from the reservoir; see “Sand Quantification: The Impact on Sandface Completion Selection and Design, Facilities Design and Risk Evaluation,” paper SPE 116713 presented at the 2008 SPE Annual Technical Conference and Exhibition, Denver, Colo., September 21-24, hereinafter referred to as “Addis (2008)”, which is incorporated by reference herein. However, this test requires sand to be produced to the surface, where all monitoring takes place. There is no guarantee that all the sand produced by the reservoir will flow to the surface. In fact, in most cases some portion of the produced sand flowing from the reservoir falls back into the well before it reaches the surface, making the test results unreliable. In addition, because all monitoring is at the surface (which is many thousands of feet away from the reservoir), there is a lengthy and unquantifiable time delay between what happens downhole and what is monitored at the surface. Furthermore, since all the control is performed at the surface, the range of flow rates and pressure drawdowns is limited. There are currently no methods for in-situ measurement of rock material properties such as grain sorting, shape, and size distribution, or measurement of the sanding potential of a reservoir formation. The lack of this information impacts the ability to design an optimal well completion, lift system, and surface facilities.